Pulse forming circuit



April 30, 1963 G. L. CLARK ETAL PULSE FORMING CIRCUIT Filed April 2o, 1961 MKM/w42 ATTORNEY United States Patent O 3,088,052 PULSE FORMING CIRCUIT v George L. Clark and .lohn J. Hickey, Hawthorne, Calif.,

assignors to Space Technology Laboratories, Inc., Hawthorne, Calif., a corporation of Delaware Filed Apr. 20, 1961, Ser. No. 104,464 7 Claims. (Cl. B15-168) cameras, which are used to record high speed transient phenomena such as plasma .shock waves, thyratron switching circuits tare used to generate high voltage rectangular gating pulses requiring extremely fast rise and decay times `of the order of nanoseconds (1 nanosecond==l )Q10-9 second). Prior attempts to reduce the rise time by decreasing the ionization time of the tubes have proceeded to the Y point of reaching the voltage land current limits of available tubes. Other approaches to the problem have resulted in reducing the rise time lat the expense of increasing the decay time.

Accordingly, a principal object of this invention is to reduce both the rise and decay times of rectangular pulses.

A further object is to provide .a pulse forming circuit cap-able of generating rectangular voltage pulses having magnitudes of several hundred volts and rise and decay times of less than 3 nanoseconds.

The foregoing and other objects .are realized 'according to the invention in a pulse forming circuit that utilizes a thynatron switching tube for controlling the discharge of current from two energy storage devices. The electrodes of the thyratr-on are comprised of -a cathode, a control grid, a primary anode, and a secondary anode. One of the energy storage devices comprises a capacitor connected to the secondary anode of the tube, while the other storage device comprises a delay line connected to the primary anode. The discharge current from both devices flows through an output load resistor connected in the cathode cincuit.

ln the quiescent state of the circuit, a negative bias voltage on the control grid maintains the tube nonconducting, such that the delay line is charged to a moderately high positive potenti-al relative to the cathode and the capacitor is charged to a potential highly positive relative to both the primary anode and the cathode.

To initiate a rectangular pulse, the tube is rendered conducting by applying a positive trigger pulse to the control grid. The capacitor quickly discharges through the tube to produce a rapidly rising output pulse. While the secondary anode remains at a higher posit-ive potential than the primary anode, some of the ldischarge current ows into the primary anode circuit in a sense to add further charge to the delay line. This charging current appears as la brie-f negative current pulse or wave that travels along the length of the delay line.

Shortly after the initiation of the traveling negative current wave, the potential of the secondary anode drops below that of the primary anode, because current flowing ice i through the tube discharges the capacitor. When this occurs, Ithe delay line discharges through the primary anode cathode circuit of the tube, thereby reversing the direction of current flow in the delay line and sending a positive going traveling current wave down the delay line. Both current waves are reflected out of phase at the end of the delay line, and are now sent back to the primary anode as a brief positive pulse tfollowed by a negative pulse.

During its discharge, the delay line delivers a constant amplitude current to the load resistance, which appears as the main portion of the rectangular output pulse. The output pulse terminates when both waves 'reach the primary anode, with the brief positive current wave reaching the primary anode slightly [ahead of the negative current wave .and serving to square up the trailing edge of the output pulse.

In the drawings, wherein like numerals indicate like pants or phenomena:

FIG. l is a schematic circuit of `one embodiment of the pulse for-ming circuit according to the invention;

RFIG. 2 is a graph showing certain voltage distributions in a thyratron tube; and

FlGS. 3, 4, and 5 are graphs of waveforms useful in explaining the operation of the pulse forming circuit.

FIG. l shows one embodiment of the pulse forming circuit according to the invention. The pulse forming circuit comprises a thyratron switching tube 10 which is preferably 4a tetrode, such as a type 2D2l. The switching tube 10 includes .a cathode 12, la control electrode 14, a primary anode lr6 surrounded by the control electrode I14, and a secondary `anode 18 spaced from the control electrode 14. As depicted herein, the switching tube 10 is connected and `operated in a nonconventional manner. For example, the control electrode @14, or the electrode which triggers the tube 10 into conduction, usually functions as a lshield electrode in conventional circuits, and the primary anode 16 usually is used to trigger the tube 10 into conduction. As will become apparent, certain advantages result from the novel openation of the switching tube 10.

A cathode load resistor Ztl is connected between the cathode 12 and ground. The control electrode 14 is biased to a highly negative potential by connection through a grid bias .resistor 22 to a negative voltage source 24. The bias on the control electrode 14 is in excess of the cutol bias of the switching tube 16. The control electrode 14 is connected to a trigger pulse source 26 through a coupling capacitor 28, the source 26 providing a voltage pulse of suiiicient magnitude to overcome the bias on the control electrode 14 and thereby cause the tube 10 to switch into a conducting condition.

The primary anode 16 is maintained at a moderately high positive potential by connection through Ia r-st volt :age dropping resistor 30 to a primary anode voltage source 32. A delay line 34 is also connected between the primary anode .16 and ground, and 'forms one discharge circuit. Y

The secondary anode '18 is maintained `at a relatively high positive potential by connection through a second yvoltage dropping resistor 36 to a secondary anode voltthe tube are realized when the bias source 24 is -75 volts, the primary anode source 32 is +800 volts, and the secondary anode source 38 is +1500 volts.

The operation of the pulse forming circuit will now be described. In the absence of a positive trigger pulse on the control electrode 14 from the trigger pulse source 26, the switching tube y10 is biased to a nonconduoting condition. As a result, the trimmer capacitor 40 is charged to the full voltage of the secondary anode voltage source 38, and the delay line 34 -is charged to the full voltage of the primary anode voltage source 32.

When a positive trigger pulse is applied to the control electrode 14 from the trigger pulse source 26, the switching tube `10 is rendered conducting, such that current ows from the secondary anode 18 to the cathode 12, to provide a low resistance discharge path for the trimmer capacitor 40. When this occurs, the primary anode 16 is immersed in the positive column, a region of the switching tube 10 extending between the secondary anode 18 and an area A next adjacent to the cathode 12, as shown in FIG. 2. For purposes of this discussion, it is not necessary to refer to the control electrode 14, hence it is omitted from FIG. 2 for convenience.

Referring in more detail to FIG. 2, which is a diagrammatic view and graph showing the voltage distribution in the space Ibet-Ween the cathode 1'2 and the secondary anode 18 of the switching tube 10, curve B represents the voltage distribution at the moment the tube 10 reaches full ionization, and curve C represents the voltage a sh'ort time later. It is seen that at the time full ionization of the tube 10 is reached, the voltage rises sharply from zero voltage at the cathode 12 to a relatively high voltage approaching the maximum secondary anode voltage 18, at the area A next adjacent to the cathode 12. From the area A -to the secondary anode y18, the voltage rises rather gradually to a voltage just below the maximum secondary anode voltage 18 before it makes a short abrupt rise to the maximum voltage. The space between area A and the secondary anode 18 is known as the positive column referred to.

Assuming that the secondary anode I18 initially is at a potential of 1500 volts and the primary anode 16 is initially at a potential of 800 volts, it is seen from curve B that at the moment the switching tube 10 lires and conducts, the primary anode l16 is at a potential, indicated at point D, that is less than that lof the positive column. Positive ions from the positive column bombard the primary anode 16 and cause secondary emission therefrom of electrons which ow to the secondary anode 18. Current therefore flows in the conventional sense from the secondary anode 18 to the primary anode 16.

Current also flows from the secondary anode 18 to the cathode 12. Thus, the trimmer capacitor `40 quickly discharges through two paths, current flowing in one path in the primary anode circuit, and in another path from the secondary anode 18 to the cathode 12 and through the load resistor 20. The current flow in the second path, through the load resistor produces a sharply rising wavefront y42 in the `output current, as shown in FIG. 3(a). In FIG. 3, the total trimmer capacitor discharge current is shown in FIG. 3(11) as a relatively large current pulse 44. The cathode current is shown in FIG. 3(c) as a smaller magnitude current pulse 46 and the primary anode current is shown in FIG. 3(d) as a current pulse 48 'of the same order of magnitude as the cathode current pulse 46. The direction of the primary anode current pulse 48 is shown negative because it is a charging current and it is assumed herein that a discharging current is positive. The direction of ow of the initial primary anode current pulse 48 is such as to add further charge to the delay line 34. Hence, during the initial flow of the tube currents, the delay line 34 does not discharge but rather is briefly charged to a higher potential.

The short pulse of primary anode current 48 owing into the delay line 34 sends traveling current and voltage waves down the delay line 34. FIG. 4 shows these waves as they would appear at a fixed point along the delay line as the waves go by. FIG. 4(a) shows the voltage e(t) as a function of time and FIG. 4(b) shows the current i(t) as a function of time. The voltage wave appears as a short positive going pulse 50 and the current wave appears as a short negative going pulse 52. The current pulse 52 is negative because it is in a direction opposite that yof the discharge current of the delay line 34, which is considered as positive.

The duration of the pulse of current -52 lasts as long as the primary anode 16 remains at a potential below that of the positive column in the tube 10. However, as the tube 10 draws current, the secondary anode 18 potential quickly falls `by an amount determined by the discharge of capacitor 40, bringing the potential of the positive column down with it and below the potential of the primary anode 16, as shown in curve C of FIG. 2. When this occurs, the current flowing in the primary anode circuit reverses direction, thereby terminating the negative current pulse 52 (FIG. 4) and initiating a discharge of the voltage in the delay line 34 through the primary anode circuit. The discharge in the primary anode circuit path gives rise to a negative going voltage wave 54 and a positive going current /wave S6 which travel down the delay line immediately behind the positive voltage wave 50 and the negative current wave 52 previously sent down the line 34. Thus the combined voltage wave 58 traveling down the line away from the primary anode 16 is a positive pulse 50 followed by a negative step 54, and the combined current wave 60 is a negative pulse 52 followed by a positive step 56. The negative voltage step 54 drops the voltage on the delay line 34 to one half of its initial value E, or to E/2. The positive current step `56 raises the current in the delay line 34 to a value I.

While the waves 58 and `60 travel down the delay line 34, the total primary anode current has the form shown in the left half lof FIG. 3(e), and the output current, as well as the output voltage, has the form shown in the left half of IFIG. 3(a).

When the traveling voltage and current waves 58 and 60 reach the end of the delay line 34 the voltage wave 58 is reflected in phase and the current wave 60 is reflected out of vthe phase with the incident waves. Thus the reflected voltage wave 58' appears as a positive pulse 50 leading a negative step 54', and the reflected current Wave 60 appears as a positive pulse 52' leading a negative step 56.

When the reected waves 58 and 60' reach the primary anode 16 again, the traveling waves terminate, the delay line 34 is fully discharged, and the output current and voltage waves terminate. However, the effect of the initial traveling current pulse '52, upon termination, is to add a positive component `61S to the output current pulse which tends to square up the trailing edge of the output pulse, as shown in FIG. 3(a). The traveling current pulse `52 is shown in FIG. 3(e) as a negative pulse in the leading edge of the total primary anode current waveform and as a positive pulse in the trailing edge thereof.

As a result of this positive component 66 the decay times of rectangular pulses having magnitudes of several hundred volts can be reduced to less than 3 nanoseconds.

Moreover, by first providing a fast discharge of the .trimmer capacitor 40 through the tube 10, the rise time can likewise be reduced to less than 3 nanoseconds. The fast discharge is enhanced by utilizing as the control electrode 14 which controls the ionization, the electrode that is conventionally used as a shield electrode. The close proximity `of one portion of the control electrode 14 to the cathode y12 permits greater control over the ionization and results in shorter ionization times with consequent fast discharge of the trimmer capacitor 40.

Typical operating values for the circuit of FIG. l are as rfollows:

Resistor 20 ohms said primary anode; a second charging circuit in- `Resistor 36 -..megohms 10 Capacitor 40 micromicrofarads 4-30 Resis-tor 30 megohms `10 Source 32 volts 800 Source 38 do 1500 Capacitor 28 micromicrofarads 100 Resistor 22 kilohms 10 Source 24 volts -75 Delay line 34 2 feet of RG-SS/U cable In one operative embodiment constructed with the above circuit values, rectangular pulses of 350 volts in magnitude were generated having rise and decay times less than 3 nanoseconds.

The value of the trimmer capacitor 40 can be varied to produce a desired waveform. The larger the value of capacitance the greater will be the spikes appearing at the beginning of the pulse and the faster the rise time. Depending upon design considerations, the capacitance may be made relatively large to produce a steep rise at the expense of overshoot in the pulse, or it may be made relatively small to produce a substantially flat waveform at the expense of longer rise time. FIG. 5 shows variations in the shape of the output waveform with variations in the value of the trimmer capacitor 40. Typically, the output waveform may have a duration of l0 nanoseconds. FIG. 5(a) shows a typical uncompensated waveform, that is, one resulting from Zero trimmer capacitance. FIG. 5(b) shows a typical properly compensated waveform resulting from the proper value of trimmer capacitance. FIG. 5(c) shows an overcompensated Waveform resulting from too high a value of trimmer capacitance.

It is now apparent that by means of the circuit arrangement of the invention it is possible to generate high voltage rectangular pulses with short rise and decay times.

The embodiments of the invention in which an exclusive property or privilege is claimed are dened as follows:

1. A pulse forming circuit, comprising: a thyratron switching tube including a cathode, a control grid, a primary anode, and a secondary anode; a load resistor connected in series with said tube; a first charging circuit including a first energy storage device connected to said primary anode; a second charging circuit including a second energy storage device connected to said secondary anode; said charging circuits including means for simultaneously charging said second energy storage device to an initial voltage greater than the voltage on said iirst energy storage device; means for maintaining said tube normally nonconductingwhereby both of said energy storage devices are fully charged to their initial voltages, and means for switching said tube into a conducting condition to provide a common discharge path for both of said energy storage devices through said load resistor.

2. A pulse forming circuit according to claim l wherein said first energy storage device comprises a delay line and said second energy storage device comprises a capacitor.

3. A pulse forming circuit according to claim l wherein said first charging circuit includes a first voltage source of given magnitude, and said second charging circuit includes a voltage source that is greater in magnitude than said first voltage source.

4. A pulse forming circuit, comprising: a thyratron switching tube including a cathode, a control grid, a primary anode, and a secondary anode; a load resistor connected in series with said tube; a iirst charging circuit including a first energy storage device connected to cluding a second energy storage device connected to said secondary anode, said load resistor being out of both of said charging circuits; said charging circuits including means for simultaneously charging said second energy storage device to an initial voltage greater than the voltage on said first energy storage device; means for applying a negative bias on said control grid that is greater than the negative cutoi potential thereof to maintain said tube normally nonconducting, whereby both of said energy storage devices are fully charged to their initial voltages; and means for switching said tube into a conducting condition to provide a common discharge path for both of said energy storage devices through said load resistor.

5. A pulse forming circuit, comprising: a thyratron switch-ing tube including a cathode, a control grid, a primary anode, and a secondary anode; a load resistor connected in series with said tube; a first charging circuit including a delay line, a first resistor, and a tirst voltage source connected in series, with said primary anode connected to the junction between sa-id rst resistor and said delay line; a second charging circuit including a capacitor, a second resistor and a second voltage source connected in series, with said second anode connected to the junction between said second resistor and said capacitor; said charging circuits including means for simultaneously charging said capacitor to an initial voltage greater than the voltage on said delay line; means for maintaining said tube normally nonconducting, whereby said capacitor and said delay line are fully charged to their intial voltages and means for switching said tube into a conducting condition to provide a common discharge path for said capacitor and said delay line through said load resistor.

6. A pulse forming circuit, comprising:

a thyratron switching tube including a cathode, a control grid, a primary anode, and a secondary anode mounted in that order in a common electron path;

a capacitor connected in the secondary anode circuit of said tube;

a delay line connected in the primary anode circuit of said tube;

means for simultaneously charging said capacitor and said delay line to initial voltages such that the voltage on said capacitor is substantially greater than the voltage on said delay line;

means for biasing said control grid to maintain said tube normally nonconducting, whereby said capacitor and said delay line are fully charged to their initial voltages;

a load resistor connected in the cathode circuit of said tube and out of the charging paths of said capacitor and said delay line; and

means for applying a trigger pulse to said control grid to render said tube conducting to form a common discharge path for said capacitor and said delay l-ine through said load resistor.

7. A pulse forming circuit, comprising:

a thyratron tube including, a cathode, a control grid, a primary anode and a secondary anode mounted in that order in a common electron stream path,

first source means for applying a posit-ive potential on said primary anode,

second source means for applying to said secondary anode a positive potential that is substantially larger than potential on said primary anode,

means for biasing said control grid to maintain said tube initially nonconducting,

a capacitor connected to said primary anode,

a delay line connected to said secondary anode,

said capacitor and delay line being initially charged to the potentials of said first and second sources respectively, when said tube is nonconducting,

7 8 a load resistor connected in series with said tube and the potential of said secondary anode falls below the out of the charging paths of said capacitor and said primary anode voltage to produce through said load delay line, resistor a fiat topped current pulse that terminates and means for applying a triggering pulse to said control When Said delay line S Completely discharged,

grid to render said tube conducting, whereupon said 5 the Charging Current PUISe hfOUglh the delay line being capacitor quickly discharges through said tube to Iefcted to Produce aTPOSmVe Plflse component produce a fast rising current pulse through said load Whlch Shafrpens the tfalmg edge 0 the load CUI" resistor and a charg-ing current pulse through said rent Wave Orm dela/ linre1 dutrmg. an tintinal 1wtlirentsad1 rimriy lo References Cited in the me of this patent v en a re a ve c alsdeegal e m po o e e o UNITED STATES PATENTS said delay line discharging through said tube when 2,487,923 Ferguson Nov. 15, 1949 

1. A PULSE FORMING CIRCUIT, COMPRISING: A THYRATRON SWITCHING TUBE INCLUDING A CATHODE, A CONTROL GRID, A PRIMARY ANODE, AND A SECONDARY ANODE; A LOAD RESISTOR CONNECTED IN SERIES WITH SAID TUBE; A FIRST CHARGING CIRCUIT INCLUDING A FIRST ENERGY STORAGE DEVICE CONNECTED TO SAID PRIMARY ANODE; A SECOND CHARGING CIRCUIT INCLUDING A SECOND ENERGY STORAGE DEVICE CONNECTED TO SAID SECONDARY ANODE; SAID CHARGING CIRCUITS INCLUDING MEANS FOR SIMULTANEOUSLY CHARGING SAID SECOND ENERGY STORAGE DEVICE TO AN INITIAL VOLTAGE GREATER THAN THE VOLTAGE ON SAID FIRST ENERGY STORAGE DEVICE; MEANS FOR MAINTAINING 